Petroleum Community Forum - Tank Venting Calculations

hello,everybody:
how to select the wetted surface of a low-pressure storage tank when itself catches a fire rather than it is exposed to a pool fire ?
when we calculate the fire water demand or tank breathing valve's venting capacity of a low-pressure storage tank with PV valves on the roof,we will decide the wetted surface of the tank.but in all of the standards or handbook,such as API RP 521,API 2000,they say:the up-limit of the wetted surface normally is 25 feet(required in API RP 521-1999 clause 3.15), or 30 feet(required in API 2000) ,the part which is higher than that up-limit may not be protected or may be not calculated when calculating the breathing valve's venting capacity.All of the up-limits are based on the assumption that the tank are exposed to an outside fire or pool fire,the pool fire's flame can not impinge higher than that up-limit.
We ignore the conflict of the value(25 or 30 feet) of different standards,we talk about why all of the standards consider the up-limit based on the assumption that the tank are exposed to an outside fire or pool fire(it means to calculate the venting capacity of adjacent tank rather than buring tank)?why not consider the burning tank itself?if a low-pressure storage tank catches a fire at its low-low level,and this tank's height is higher than 30 feet ,how to select its wetted surface when calculating breathing valve's venting capacity?
in API 2000,table 3B footnote a,it says"Engineering judgment is to be used in evaluating the portion of the area exposed to fire."
so,for the buring tank,the whole tank height shall be considered.
but,why all of the standards consider the tank venting capacity based on the assumption of exposing to a pool fire?why not consider the buring tanks' venting?

Originally Posted by Nephilim

There is no conflict between API 521 and 2000, API 521 is not applicable to storage tanks.

How could a storage tank catch fire and would venting actually still matter?

Btw, both API standards you are referring to have been replaced / updated. They're now ISO standards as well.

Originally Posted by taoxianwen123

in NFPA FIRE PROTECTION HANDBOOK(19th edition),page 6-302 says:
The required flow rate permitted by NFPA 30 to be multiplied
by an appropriate factor F listed in the following schedule
when protection is provided as indicated (only one factor may be
used for any one tank):
• 0.5 for approved drainage for tanks over 200 sq ft (18.6 m2)
of wetted area
• 0.3 for approved water spray and drainage
• 0.3 for approved insulation
• 0.15 for approved water spray with approved insulation and
drainage
With some restrictions, the above factors may be reduced
by 50 percent for liquids with heats of combustion and rates of
burning equal to or less than those of ethyl alcohol (ethanol), but
in no case can the factors be reduced to less than 0.15. See
NFPA 30.
but all of the vent capacity is based on the assumption that the tank is exposed to a outside fire, how to calculate the vent capacity or venting flowrate of buring tank?

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